The present invention relates to fluorescent lamps, and more particularly, to ballast circuits for fluorescent lamps including a self-oscillation circuit having a high power factor and an end-of-lamp-life protection circuit.
In the lighting of fluorescent lamps, a gas enclosed within a glass tube is caused to become ionized, thus reducing a breakdown voltage between electrodes placed at opposite ends of the glass tube. Ionization is initiated by heating of the electrodes. Once the gas is sufficiently ionized, a voltage at or above the breakdown voltage is placed across the lamp electrodes to thereby cause a current arc to form across the electrodes. The arc produces a bright glow within the lamp tube and produces radiation that activates a fluorescent coating on the inner surface of the glass tube, to thereby produce a bright light.
In controlling the turning on and off of fluorescent lamps, it is necessary to control the current to the lamp and to provide a starting voltage. In fluorescent lamps, this task is performed by a circuit called a ballast, also referred to as a ballasting circuit. There are generally two types of ballasts: magnetic ballasts and electronic ballasts.
Presently, most low wattage fluorescent lamps utilize magnetic ballasts that include magnetic chokes or suitable magnetic transformers and glow bulb starters. The magnetic choke limits current flow to the lamp while the glow bulb starter creates a voltage spike across the lamp after sufficiently preheating the electrodes. These magnetic ballasts are considered inefficient because of considerable power dissipation in the magnetic components. Moreover, these ballasts exhibit low power factors because of the highly inductive reactances of the magnetic chokes. The power factor is the ratio of the average (or active) power to the apparent power (root-mean-square [rms] voltages times rms current) of an alternating circuit.
Further, the glow bulb starters associated with these ballasts exhibit random starting times that produce unpleasant flashes or flickering as an arc attempts to be established across the electrodes of the lamp. This is especially true at low line voltages because the ballasts permit too much voltage to be applied to the bulbs, due to the inadequacies in the ballast design. Arcs are then produced across the bimetal components of the bulbs as the voltage will be nearly high enough to sustain arcing, and annoying flickering and restriking occurs. As a result, the performances of glow bulb starters are not predictable and this results in unreliable starting times of the fluorescent lamps.
Electronic ballasts are very expensive and can suffer from poor reliability due to the larger number of components involved. In these ballasts, a variety of electronic components are utilized to heat up the electrodes of the lamp and to establish the breakdown voltage across the electrodes. In addition, in conventional electronic ballasts a large number components, including integrated circuit components, are required to control the power factor of the conventional electronic ballasts (i.e., See Wang et al., U.S. Pat. No. 6,300,723).
Magnetic ballasts have reliability problems after 6,000 cycles because of contact wearout in the associated glow bulb starters therewith. Electronic ballasts suffer from similar reliability problems because of the larger number of discrete components used.
In addition, to meet Underwriters Laboratory, Inc. safety standards for current leakage of an electronic ballast while replacing a lamp (i.e., relamping), there is a need for an electronic circuit that is able to sufficiently protect against current leakage during relamping without using a large number of components and while still having a high power factor correction.
To overcome the above identified problems of a conventional ballast circuit, a ballast system consistent with the present invention is provided that has an improved power factor resulting in a more efficient operation of a lamp. Furthermore, the ballast system also detects end-of-lamp-life of a lamp to protect against high voltage conditions that may occur as the lamp fails to draw sufficient current.
In accordance with articles of manufacture consistent with the present invention, a ballast system for use with a fluorescent lamp having two filaments disposed at opposite ends of the lamp is provided. The ballast system includes a DC input terminal for connection to a DC voltage source or for receiving a rectified DC signal, a capacitor operably connected between the DC input terminal and the lamp, and an inductor. The lamp operably connects the capacitor in series with the inductor. The ballast system also includes a switching means that is operably connected to the DC input terminal and to the capacitor for sensing a change in voltage across the inductor and for controlling current from the DC voltage source to the capacitor in response to the change in voltage across the inductor. The switching means controls the current to the capacitor such that the current has a waveform and a frequency that is preferably approximately equal to a series resonant frequency defined by the capacitor and the inductor.
The ballast system may also include an electronic starter circuit operably connected between the switching means and the DC input terminal such that the electronic starter circuit triggers the operation of the switching means when the predetermined voltage level is present on the DC input terminal.
The ballast system may also include a startup capacitor operably connected between the two filaments of the lamp and a startup resistor operably connected in parallel to the startup capacitor. In this implementation, the ballast system may further include an end-of-lamp-life sensor operably connected across one of the two filaments of the lamp. The end-of-lamp-life sensor is operably configured to detect when a second predetermined voltage level is present at one end of the one filament of the lamp and to momentarily substantially short the one filament causing a pulse with a predetermined magnitude to be sent through the inductor to the switching means when the second predetermined voltage level is detected.
The ballast system may further include an end-of-lamp-life cutoff circuit and a transformer that has the inductor as a primary winding and that has a secondary winding. In this implementation, the end-of-lamp-life cutoff circuit is operably connected to the secondary winding and to the switching means. The end-of-lamp-life cutoff circuit also has means for causing the switching means to inhibit current flow to the lamp when the pulse with the predetermined magnitude is sent through the inductor and sensed via the secondary winding.